Implement transaction json serialization/deserialization

[pananton]

SDK currently has json::Parser which allows to read/write any variadic struct. We need to describe format of json-serialized transaction using variadic struct for all transaction types and add functions that convert this DTOs to transaction classe and viceversa.

[eugene-babichenko]

Will you prepare only transactions serialization, or also other data types?

[pananton]

This issue is only for transaction types and types required by them. When this issue will be closed, I probably will have a time to work on adding other API types and endpoints.